System and method of operation for communication by television



Nov. 24, 1936.

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Patented Nov. 24, 1936 UNITED STATES SYSTEM AND LIETHOD OF OPERATION FOR COMJWUNICATION BY TELEVISION William A. Tolson, Erlton, N. J., assignor to Radio Corporation of America, a corporation oi Delaware Application June 25, 1931, Serial No. 546,782

Claims. (01. 178-695) My invention relates to improvements in systems and methods of operation for communication by television, and more particularly to improved systems and methods for synchronizing 5 operations at the transmitting and receiving stations.

In establishing communication by television, it is essential that synchronism be maintained between operating actions at the transmitting and receiving stations. Among the various systems proposed heretofore, and which have had some degree of success as regards synchronization, are those depending upon filters at the receiving station for separating the received picture and the synchronizing signals.

These filters are expensive and complicated and, moreover, the systems employing them re quire channels or frequency bands additional to those required for the picture signals. In the systems referred to, furthermore, the filtering action at the receiving station has not been entirely effective to prevent the synchronizing signals from affecting, in some degree, the fidelity of the picture.

With the foregoing in mind, it is one of the objects of my invention to provide an improved system and method for communication by television wherein the required synchronization is obtained without recourse to the use of any filters,

as heretofore, and wherein the picture and synchronizing signals are transmitted in the same channel and utilized at the receiving station for their respective purposes without the received synchronizing signals interfering in any way with the fidelity of the picture as determined by the picture signals.

Another object of my invention is to provide an improved system and method of operation for communication by television having advantages over systems and methods proposed heretofore for such purpose as regards simplicity, reliability and steadiness of operation, and picture fidelity.

Other objects and advantages will hereinafter appear.

In accordance with my invention, a synchronizing signal or electrical effect is transmitted once for each picture-frame period, and the scanning and framing actions at the receiving station are both synchronized, with operating action at the transmitting station, solelyv by these signals or effects as the same are intercepted at the receiving station.

Further, in accordance with my invention, a synchronizing wave of a given frequency is transmitted, the received wave is rectified at the receiving station to develop therefrom electrical impulses, and such impulses are utilized to control operating action at the receiving station.

Further, and-more specifically, in accordance with my invention, a synchronizing wave of a 5 given frequency is transmitted during and only during periods intervening the picture-frame periods, thereceived wave is rectified at the receiving station to develop therefrom electrical impulses, and such impulses are utilized solely to 10 synchronize scanning and framing actions at the receiving station with operating action at the transmitting station.

My invention resides in a system and method of operation of the character hereinafter de- 15 scribed.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawings, wherein Figure l is a fragmentary view, partly diagram- 2- matic, of a television transmitting station, illustrative of the means for generating synchronizing signals.

Fig. 2 is a graphical illustration of the radio spectrum of the transmitted picture and syn- 25 chronizing signals;

Fig. 3 is a diagrammatic view of a television receiving station embodying my invention;

Fig. 4 is a graphical illustration bearing upon the principle of operation of the receiving station 30 shown in Fig. 3;

Fig. 5 is a diagrammatic view of the vertical deflection apparatus illustrated in Fig. 3;

Fig. 6 is a diagrammatic view of the horizontal Fig. '7 is a view similar to Fig. 5, showing a .proposed modification; and

Fig. 8 is a graphical illustration bearing upon the principle of operation of the apparatus shown in Fig. 7. v 40 At the transmitting station, a suitable 30 kc. oscillator l0 supplies a synchronizing tone or wave of this frequency to a radio transmitter H which may be the same as that used for transmitting the picture signals. 45

The oscillator I0 is adjusted to oscillate only when a suitable switch I2 is closed. This switch is closed by a cam portion l3 on the edge of a scanning disc I4 provided with the usual spirally arranged scanning holes IS. The length of the 50 cam portion l3 and the adjustment of the switch l2 are such that the 30 kc. synchronizing wave is generated and transmitted at periods in between the picture-frame periods, for which purpose one or more of the scanning holes at an end 55 of the spiral are plugged or omitted. No picwave is accordingly transmitted in the same channel with the picture signals.

In the present description of an embodiment of my invention, it is assumed that there are sixty scanning holes l5, and that the disc l4 rotates at the rate of 1200 R. P. M.

With reference now to Fig. 3, the intercepted picture and synchronizing signals appear in resistors l6 and I 1 connected across the output line 18 from a suitable radio receiver IS.

The picture signals are taken from the resistor l6 by an adjustable contact 20 and applied to the control grid 2| of a cathode ray tube 22 of the general type disclosed in the copending application of Vladimir K. Zworykin, filed November 16, 1929, and bearing Serial No. 407,652.

- The cathode ray 23 is caused to scan the usual fluorescent screen 24 at the large end of the tube.

For this purpose, horizontal deflecting plates 25 I are connected across the output lines 26 from suitable apparatus 21 which generates a 1200 cycle saw-tooth voltage wave for effecting horizontal deflection of the ray 23. Electromagnetic coils 28, for effecting vertical deflection of the ray, are connected across the output lines 29 from suitable apparatus 30 which generates a 20 cycle saw-tooth voltage wave.

For the purpose of synchronizing operating actions of apparatus 21 and 30 with operating action at the transmitting station, the received 30 kc. Wave is taken from the resistor 11 by an adjustable contact 3| and applied to the grid of a tube 32' in the plate circuit of which is a tuned circuit 33 adjusted to resonate at the frequency of '30 kc.. The tube 32 operates as a selective amplifier to amplify the 30 kc. frequency and reject all other frequencies.

The tube 32 serves to drive the grid circuit of a tube 34 biased to function as a rectifier.

The 30 kc. wave applied to the grid of the tube 34 results in increase in its D. C. plate current. The voltage Ep appearing in the plate circuit of v the tube 34 is, therefore, a flat-topped wave or intermittent surge of the general character shown in Fig. 4, and having a frequency equal to the number 01' revolutions per second of the scanning disc l4 at the transmitting station. The voltage impulses 35, therefore, are separated by periods of one twentieth of a second.

The apparatus 30 for vertical deflection is connected to the tube. 34 by connections 36 and 31, while the apparatus 21 for horizontal deflection is connected to this tube by connections 38 and 39.

With reference to Fig. 5, the apparatus 30 comprises, more particularly, a condenser 40 continu-. ously charged through a high resistance 4| from a high positive potential source of 1,000 volts, as indicated. Z I

When the voltage across the condenser 40 builds up to the breakdown voltage of an associated grid-glow tube 42, the latter becomes conductive to discharge the condenser practically instantaneously. The capacity of the condenser 40, the charging potential of 1,000 volts therefor,

the value of the resistance 4|, and the adjustment of the tube.42 are such as to causethe condenser 40 to charge and discharge at a rate substantially below the desired picture frequency of twenty times a, second. The saw-tooth voltage wave in the plate circuit of tube 42 is impressed upon the grid of an amplifier tube 3, the amy the transmitter.

plified saw-toothv voltage wave in the plate circuit of this tube causing a similar saw-tooth current wave to pass through the coils 28.

The apparatus 30'is synchronized by causing the tube 34 to drive the tube 42. That is, each of the synchronizing impulses in the plate circuit of the tube 34 is effective, when applied to the grid of the tube 42, to cause the latter to discharge the condenser 40. It will, therefore, be seen that, at the beginning of each picture-frame period, the condenser begins to charge from the same point on its charging curve.

With reference to Fig. 6, the horizontal deflection apparatus 21 comprises, more particularly, a dynatron oscillator 43a having connected in its plate circuit an adjustable oscillating circuit 44 which is adjusted to substantially therequired frequency of 1,200 cycles. In operation, this adjustment is determined by the occurrence of a slow drift of the received picture as observed on the fluorescent screen 24 of the tube 22.

The apparatus 21 is synchronized by causing the tube 34 to apply the synchronizing impulses 35 to the grid of theoscillator 43a. These impulses are effective, in the well-known manner, to lock the oscillator in step with the transmitting disc l4. It has been determined that if the free oscillating period of the tube 43a is synchronous with the transmitting disc to within one quarter cycle per picture, the impulses 35 are effective to synchronize the oscillation of this tube with scanning action at the transmitter. It has also been found that the adjustment of the circuit 44, for this purpose, is easily maintained.

A condenser 45 is charged through a resistance 46 from a potential source of 250 volts, as indicated, and is connected in the plate circuit of a screen-grid tube 41. The tube 41 is biased below cut-off to a point whereat only the peaks of the voltageimpulses in the plate circuit of the tube 43a are effective to drive the grid circuit of the tube 41. These impulses on the grid of the tube 41 serve to discharge the condenser 45 at the rate of 1,200 times a second, the discharge rate being somewhat greater than ten times the charging rate. i

From the foregoing it will be understood that a 1,200 cycle saw-tooth voltage wave is developed in the plate circuit of the tube 41. This wave is amplified by suitable amplifier tubes 48 and 49, and the amplified wave impressed, by way of a suitable step-up transformer 50, across the plates 25. By interposing the transformer 50 between the plates 25 'and the tube 49, it is possible to use on the latter a plate voltage substantially lower than would otherwise be necessary.

The received synchronizing wave, as explained, is utilized to effect practically instantaneous discharge of the condensers 40 and 45. This action, therefore, provides that at the beginning of each picture period, the cathode ray 23 is directed at the same spot on the screen 24.

An important advantage of my improved system and method for synchronization resides in v the fact that only one synchronizing signal per picture is required. This permits the transmission of the synchronizing signal during the intervals between successive pictures, and thus imposes no sacriflce of modulation percentage at Furthermore, in my improved system and method, it is not necessary to em ploy band elimination filters at either the transmitter or receiver. This feature, therefore, slim inates the very troublesome problems of avoiding phase-shift in the transmitted and received signals when band elimination filters are necessary, as in the various systems and methods proposed heretofore.

In lieu of the apparatus 30 for vertical deflection, it lies within the scope of my invention to use, when desired, apparatus 30a in Fig. 7, which embodies the so-called Van der Pol fourelectrode tube relaxation oscillation circuit. In this apparatus, a screen-grid tube 5| is connected, as shown, to function as a relaxation oscillator, the screen-grid voltage curve having the shape shown in Fig. 8. This wave, it will be noted, is very nearly saw-tooth in shape, with a very pronounced reversed transient at the end of each cycle. This voltage wave shape is effective to force a sawtooth current wave throughan inductive circuit, for which purpose the wave is amplified by a suitable amplifier tube 52 in the plate circuit of which the coils 28 are connected.

The synchronizing impulses 35 areapplied by the connection 36 to the grid of the tube 5| and drive the same in the wellknown manner; The apparatus 30a is thereby synchronized, and the saw-tooth current wave through the coils 28 is maintained at 20 cycles.

The various values of capacity, resistance, inductance and voltage indicated on the dra'ings have been found to be satisfactory for successful operation. These values, however, are not critical in any strict sense of the word, and may be varied over a substantial range to suit particular requirements.

Furthermore, it will be understood that other changes may be made, such as in the number of the scanning holes I5, the R. P. M. of the scanning disc I4, and the frequency of the synchronizing wave without departing from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In a television system, means for developing picture signals during spaced periods of time,-

means for developing a synchronizing tone at a relatively high frequency as compared with the frequency of occurrence of said periods means for transmitting said signals and tone together with the latter occurring only substantially during the periods intervening said first-named periods, scanning apparatus and framing apparatus forming part of a receiving station for effecting respectively scanning and framing actions thereat, means forming part of the receiving station for rectifying the received synchronizing wave to develop an electrical impulse once for each picture-frame period, and means for synchronizing said scanning and framing apparatus with operating action at the transmitting station solely by such impulses.

2. In the art of television between transmitting and receiving stations, the method of operation which comprises transmitting a synchronizing wave of a given frequency during and only during periods intervening the picture-frame periods, said frequency being high compared with the frequency of occurrence of said picture-frame periods, rectifying the received wave at the receiving station to develop therefrom electrical impulses, and utilizing solely such impulses to synchronize both scanning and framing actions at the receiving station with operating action at the transmitting station.

3. In the art of television, the method of operation which comprises developing picture signals during spaced periods of time, developing a synchronizing tone at a relatively high frequency as compared with the frequency of occurrence of said periods, and transmitting said signals and tone together with the latter occurring only substantially during the periods intervening said first-named periods.

4. In a television transmitter, means for developing picture signals during spaced periods of time, means for developing a synchronizing tone at a relatively high frequency as compared with the frequency of occurrence of said periods, and means for transmitting said signals and tone together with the latter occurring only substantially during the periods intervening said first-named periods.

5. In a television receiver, a cathode ray tube provided with a screen and with means for de-,

veloping a ray of electrons and directing the ray at said screen, means for deflecting said ray in one direction, meansfor deflecting said ray in a direction substantially perpendicular to said first-named direction, means for intercepting an incoming synchronizing tone, a circuit supplied from said last-named means and tuned to said tone, and rectifying means supplied from said circuit and operating to rectify said tone to produce synchronizing impulses, said rectifying means being common to said first and secondnamed deflecting means and operating to supply the same individually with said impulses, said first and second-named deflecting means being responsive to and controlled by said impulses. WILLIAM A. TOLSON. 

